The Islamic State group claimed responsibility for the attack.
Media reports said a truck loaded with large quantities of explosives blew up on the western edge of the town of Qamishli, followed by an explosives-packed motorcycle a few minutes later in the same area.
The blasts caused massive damage in the area and rescue teams were working to recover victims from under the rubble, the SANA news agency said.
Qamishli, near the Turkish border, is mainly controlled by Kurds but Syrian government forces are present and control the town's airport.
Qamishli resident Suleiman Youssef, a writer, told The Associated Press by telephone that he heard the first explosion from few miles away. He said the blasts leveled several buildings to the ground and many people were trapped under the rubble.
"Most of the buildings at the scene of the explosion have been heavily damaged because of the strength of the blast," he said.
The Islamic State group, in a statement published by the IS-linked Aamaq news agency, said it carried out the attack in Qamishli, describing it as a truck bombing that struck a complex of Kurdish offices. The extremist group has carried out several bombings in Kurdish areas in Syria in the past.
Today's explosion came as US-backed Kurdish forces pressed ahead with their offensive to take the IS-held town of Manbij, also in northern Syria but further to the east of Qamishli.
